1. Field of the Invention
The present invention relates to a barium titanate semiconductor ceramic powder and to a laminated semiconductor ceramic device having a positive resistance-temperature characteristic provided with a semiconductor ceramic layer formed by sintering the barium titanate semiconductor ceramic powder.
2. Description of the Related Art
Barium titanate semiconductor ceramics can have positive resistance-temperature characteristics (positive temperature coefficient characteristic or PTC characteristic) in which resistivity at room temperature is low and resistance rapidly increases above a certain temperature (Curie temperature). They are widely used in temperature control and current control and the like devices. Among these, an overcurrent protection device is desired to be compact, to have a high breakdown voltage, and specifically, to have lower resistance at room temperature.
As a device responding to these desires, a laminated semiconductor ceramic device is proposed in, for example, Japanese Laid-open Patent Application No. 57-60802. The laminated semiconductor ceramic device is obtained by alternately laminating semiconductor ceramic layers primarily composed of barium titanate with internal electrodes composed of a platinum-palladium alloy (Pt-Pd alloy) followed by firing. By employing the laminated structure, the area of the internal electrodes in the semiconductor ceramic device can be greatly increased, and hence, miniaturization of the device can be realized.
However, it is difficult to obtain ohmic contact between the internal electrodes and the semiconductor ceramic layers in the laminated semiconductor ceramic device since a Pt-Pd alloy is used as the internal electrodes, and hence, there is the problem in that resistance at room temperature greatly increases.
As a material used for the internal electrodes instead of the Pt-Pd alloy, a nickel-containing metal (hereinafter referred to as Ni-based metal), such as nickel or an alloy containing nickel, is proposed in, for example, Japanese Laid-open Patent Application No. 6-151103. An internal conductive material composed of a Ni-based metal and the semiconductor ceramic show superior ohmic contact therebetween, and as a result, an increase of resistance at room temperature can be prevented. However, the Ni-based metal forming the internal electrodes is readily oxidized in a general firing step in the air. Consequently, after a firing step in a reducing atmosphere is performed, a re-oxidizing step is required to take place at a relatively low temperature so that the Ni-based metal is not oxidized. This causes the problem that resistance-change range is reduced to be lower by two orders of magnitude. Accordingly, the breakdown voltage thereof is not sufficient, and there is a problem in practical use.
Accordingly, an object of the present invention is to provide a compact and low resistance laminated semiconductor ceramic device having positive resistance-temperature characteristics, in which the laminated semiconductor ceramic device exhibits sufficient change in resistance and a high breakdown voltage in addition to ohmic contact between semiconductor ceramic layers and internal electrodes.
Another object of the present invention is to provide barium titanate semiconductor ceramic powder which is advantageously used for forming the semiconductor ceramic layers provided in the laminated semiconductor ceramic device described above.
To these ends, the inventors of the present invention found through long intensive research that a compact and low resistance laminated semiconductor ceramic device having a sufficient change in resistance and a high breakdown voltage can be obtained by using barium titanate semiconductor ceramic powder provided with certain specified properties.
That is, the barium titanate semiconductor ceramic powder of the present invention has an average particle diameter of not more than about 1.0 xcexcm, a c/a axis ratio of not less than about 1.005, and a barium (Ba) site/titanium (Ti) site ratio from about 0.99 to 1.01, in which a donor element is dissolved.
Even though the barium titanate semiconductor ceramic powder of the present invention can be synthesized by various synthetic methods, when the barium titanate semiconductor ceramic powder is synthesized by a hydrolysis method, the Ba site/Ti site ratio is preferably from about 0.99 to 1.00, and the barium titanate semiconductor ceramic powder is synthesized by a solid phase method, the Ba site/Ti site ratio is preferably from about 1.00 to 1.01.
The present invention can be applied to a laminated semiconductor ceramic device provided with a plurality of internal electrodes and a plurality of semiconductor ceramic layers alternately laminated with the internal electrodes. In the laminated semiconductor ceramic device, according to the present invention, the semiconductor ceramic layers can be obtained by sintering the barium titanate semiconductor ceramic powder described above.
In the laminated semiconductor ceramic device described above, the internal electrodes is preferably composed of a conductive component containing a Ni-based metal, that is, a nickel-containing metal, such as nickel or a nickel-containing alloy.